Electronic technology, with its associated solid state components, has evolved into the miniaturization of sensitive large scale integrated circuits used to develop sophisticated and low power electronic products for both consumer and industry. At least some of these devices, including particularly CMOS and MOSFET devices, are sensitive to damage and degradation from localized static charges, that can occur, for example, during packaging, assembly, and field installation. By way of specific example, it has been found that walking across a carpeted area can generate enough static voltage to destroy some CMOS devices, and statically charged, non-conductive plastics can present a field hazard when the charge is as little as 500 volts.
Static charge elimination, or at least reduction, during manufacture of sensitive systems, has been the target of considerable research as well as product development. Also, in the past few years, many papers have been written on the subject of electrical overstress and electrostatic discharge, and various symposiums and technical papers have been directed thereto.
Numerous active and passive types of equipment, ranging from complete room ionization systems to bench top products, have heretofore been suggested and/or utilized in an attempt to control static discharge. The active products essentially use the same general principle for minimizing or eliminating static charges, but utilize different techniques.
The application of the general principle normally utilized to control static charges consists of a means of generating equal and sufficient amounts of positive and negative air ions, and then propelling them into a neutralizing, or work, area in order to discharge any charged materials thereat.
Radioactive materials have heretofore been used for ion production with such radioactive materials producing alpha particles with sufficient energy to collide with neutral air molecules and dislodge electrons from their outer orbits. This can produce a nitrogen or oxygen molecule with one less electron than normal thereby creating a positive ion. The dislodged electron with a charge of about 1.6.times.10.sup.-19 coulombs attaches itself to another neutral molecule and becomes a negative air ion. The isotope used to generate these ions have a short half life and must be replaced every six months to one year.
The radioactive system to generate ions requires a fan or blower since the ions will travel only between two and four inches from the radioactive source. The fan blows a turbulent flow of air through the positive and negative ions and propels them into the work area. The effective working distance of this system is related to how far the ions can be propelled before recombination occurs. Therefore, the larger the fan, the more cubic feet of air, and the faster and therefore farther the ions are propelled.
A second arrangement heretofore utilized to produce ions utilizes electrical means whereby a high voltage AC power supply is attached to a sharp needle point which intensifies the field surrounding the needle. The same mechanisms that produce the ions using a DC power supply, as brought out hereafter, apply to the AC power supply system. However, since the AC system voltage changes polarity at about 60 HZ intervals, both positive and negative ions can be produced from a single needle source.
The AC system to generate ions also requires a fan or a blower to propel the produced ions toward the work area since the 60 HZ line frequency used to generate the ions propels the electrons from the sharp needle point on the negative half of the cycle, and removes electrons from the surrounding air on the positive half of the cycle. This will result in ion generation that will be transported only about two to four inches from the needle source depending on the amplitude of the voltage. The fan blows a turbulent flow of air across a series of sharp needles and propels the ions into the work area. The effective working distance of this system is the same as described for the radioactive system. However, a long series of needles spaced at an appropriate distance can be suspended from the ceiling of a room, and gravity used to fill an entire room with oppositely charged ions.
A third arrangement (which is the type arrangement used in this invention) heretofore utilized to produce ions utilizes electrical means whereby a DC high voltage power supply is attached to a share needle point which intensifies the field surround the needle. The dielectric strength of air is overcome, corona discharge occurs, and current flows either into the needle point from the air for positive ions, or from the needle point into the air for negative ion generation. The field strength needed depends upon temperature and pressure and is generally between 20,000 and 30,000 volts per centimeter. Since it is generally easier to produce negative ions than positive ions, the positive power supply is usually adjusted to a higher DC potential than the negative supply to create the same number of ions.
The DC voltage system to generate ions requires at least two sharp needle points spaced at an appropriate distance with opposite polarity power supplies (generally under 10,000 volts each) in order not to exceed OSHA ozone limits of 0.1 ppm. The DC voltages utilized have also been pulsed either into the two needle points, or a single point may be used if the positive and negative voltages are alternately switched into the single point.
The DC voltage system of ion generation has used several methods to propel the ions into the work area. Since two independent needles are used, one to produce positive ions and the other to produce negative ions, electric fields of opposite polarities are generated at the needle points.
At the negative needle point, a constant source of electrons from the needle point are propelled into the air in front of the needle. Since like charges repel each other, the electrons are propelled by repulsion into the air, as are the negative air ions generated by corona discharge in the vicinity of the needle point.
At the positive needle point, electrons are pulled out of the surrounding air and positive ions are generated by corona discharge in the vicinity of the needle point. Again, the like charges repel each other and the positive ions are propelled by repulsion into the air.
If the discharge or needle points are closer spaced than about three to four feet, ion current will also flow between these electrodes. The magnitude will be related to the square of the distance between the electrodes. Also in the area, the positive ions will be attracted to the negative ions and recombination will occur.
The foregoing results in a constant source of positive and negative ions propelled thru the air by ion repulsion without the aid of a fan or blower. If the DC voltages at the needle electrodes are pulsed, the ions can be propelled even further distances than with bipolar constant DC. The increased propulsion distance will be related to the pulse time and is typically about two to four seconds. However, as the pulse frequency decreases, spurts of alternate polarity ions can charge up isolated conductors or non-conductors to several thousand volts for this two to four second period of time in close proximity to the pulsed DC equipment. This can be dangerous to sensitive electronic equipment.
A fan or blower has also been used to propel the ions generated by DC techniques even further into the work field. Again, the fan has been heretofore used to blow a turbulent flow of air across closely spaced electrodes of opposite polarity, either constant DC or pulsed DC. With pulsed DC systems the pulse time is usually decreased from a time of two to four seconds to 1/4 to 1/2 second in order to reduce the spurts of alternate polarity ion charge concentrations that may be dangerous to sensitive electronic equipment. Thus, bipolar constant DC or pulsed DC systems can be used as total room air ionization systems without the use of a fan by suspending the needle emitters with appropriate spacings at the ceiling.
Static charge control devices having both positive and negative needle electrodes for producing ions are shown, for example, in U.S. Patents issued to Moulden (U.S. Pat. Nos. 4,319,302 and 4,333,123, for example), and in U.S. Patents issued to Saurenman (U.S. Pat. No. 3,624,448, for example), with the needle electrodes being pulsed by means of a voltage generator coupled to the needle electrodes. In the device shown in the referenced Moulden patents, the needle electrodes are positioned within plastic tubes, and in the device shown in the referenced Saurenman patent, the needle electrodes are positioned within shaped recesses.
Utilization of forced air units, such as a fan, to propel ions away from an area where ions have been produced, is also shown, for example, in U.S. Pat. Nos. 4,319,302, 4,333,123 and 3,624,448. Not all systems heretofore suggested, however, have required forced air units, and a system that does not utilize forced air is shown, for example, in U.S. Pat. No. 4,038,383 (Breton).
Balancing of ions directed to a work area has also been heretofore suggested, with balancing by adjusting the positioning of the needle electrodes being shown in U.S. Pat. No. 4,092,543 (Levy), for example, which patent also suggests that the prior art teaches such balancing by adjustment of the DC voltages supplied to the needle electrodes.
As can be appreciated from the foregoing, while various devices have heretofore been suggested for controlling static charges, improvements in such devices, including improvements in directing ions away from the ion producing area, in providing of voltages to the electrodes, and/or in positioning of the elements of the system, can still be utilized.